The coupling between electron transport and phosphorylation in respiratory and photosynthetic systems occurs through a current of protons flowing in a circuit in which the electron transport chain acts as a proton pump, and the membrane-bound ATPase behaves as a protochemical cell, driven in reverse by the protonic potential. An understanding of the mechanisms of electron transport linked proton pumps requires detailed knowledge of the components involved and their thermodynamic and kinetic characteristics. Photosynthetic bacteria, which have an electron transport chain similar to that of mitochondria, have proved particularly useful for kinetic studies of the relation between electron flow, proton flux and electrogenic events. We propose to extend our studies of these systems in three main areas: 1) Our kinetic studies show interactions between components from different sections of the chain. Depletion of specific components using biochemical methods or mutant strains will enable us to probe these integrated redox cycles. We will also investigate the kinetics of pseudo-steady-state transients to follow the detailed reaction rates between components. 2) Automated redox potentiometry will enable us to resolve all spectrophotometric components in a single small sample, and will be used for detailed thermodynamic studies, and for characterizing electron transport mutants. 3) Electrochromic shifts of the spectra of photosynthetic pigments can be used as indicators of membrane potential. Spectrophotometric and biochemical resolution of the populations of pigments involved are needed before we can understand the mechanism of these changes.